RFR: 8257876: Avoid Reference.isEnqueued in tests [v3]

[Kim Barrett]

> Please review this change that eliminates the use of Reference.isEnqueued by
> tests.  There were three tests using it:
> 
> vmTestbase/gc/gctests/ReferencesGC/ReferencesGC.java
> vmTestbase/gc/gctests/WeakReferenceGC/WeakReferenceGC.java
> jdk/java/lang/ref/ReferenceEnqueue.java
> 
> In each of them, some combination of using Reference.refersTo and
> ReferenceQueue.remove with a timeout were used to eliminate the use of
> Reference.isEnqueued.
> 
> I also cleaned up ReferencesGC.java in various respects.  It contained
> several bits of dead code, and the failure checks were made stronger.
> 
> Testing:
> mach5 tier1
> Locally (linux-x64) ran all three tests with each GC (including Shenandoah).

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into no_isenqueued
 - move REMOVE and RETAIN decls and init
 - update WeakReferenceGC test
 - update ReferenceQueue test
 - update ReferencesGC test

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1691/files
  - new: https://git.openjdk.java.net/jdk/pull/1691/files/01710567..d5355342

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1691&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1691&range=01-02

  Stats: 7952 lines in 328 files changed: 5091 ins; 1646 del; 1215 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1691.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1691/head:pull/1691

PR: https://git.openjdk.java.net/jdk/pull/1691